The present invention relates to sense amplifiers used to sense data in CMOS memory cells, and more specifically, to a latch control circuit within such a sense amplifier.
In an integrated memory circuit, sense amplifiers are used to improve the speed performance of a memory, and to provide signals which conform with the requirements of driving peripheral circuits within the memory. A sense amplifier is an active circuit that reduces the time of signal propagation from an accessed memory cell to the logic circuit at the periphery of the memory cell array, and converts the arbitrary logic levels occurring on a bitline to the digital logic levels of the peripheral circuits. The sensing part of the sense amplifier detects and determines the data content of a selected memory cell. The sensing may be xe2x80x9cnondestructivexe2x80x9d, wherein the data content of the selected memory cell is unchanged, such as in SRAMs, ROMs and PROMs, or the sensing may be xe2x80x9cdestructivexe2x80x9d wherein the data content of the selected memory cell may be altered by the sense operation, such as in DRAMs.
Many sense amplifiers tend to have a voltage swing on the output. This is because an effective gate voltage on the input of the circuit that is driven by the sense amplifier results in faster output switching times in the driven circuit, but the switching of a greater effective gate voltage and a larger charge on the same load capacitance, with the same output current, requires a longer switching time. In order to improve speed and power performances of sense amplifiers, it is known in the art to limit the amount of voltage swing to a small optimized level. In addition to substantial improvements in speed and power, the reduction of voltage swings becomes critical in designs for deep-submicrometer CMOS technologies. Reduced voltage swings results in a decrease in hot-carrier emissions, cross-talkings, noise, and operation margin degradation. For output voltage swing limitation, the most widely used techniques are the amplitude timing technique and the voltage clamping technique. The amplitude timing technique is implemented by deactivating the sense amplifier at the time point when the voltage swing is at the optimum level. However, this technique may result in large variations of the voltage swing due to device parameter changes. The voltage clamping technique is less prone to device parameter fluctuations.
As the trend towards smaller size memory devices continues, it is desirable to control the voltage swing of the sense amplifiers using the smallest number of transistors possible, and using transistors having a small size, i.e. a small W/L ratio. Additionally, to meet the demand for greater speed, it is desirable for the sense amplifier to operate as fast as possible, while still maintaining a controlled voltage swing.
It is the object of the present invention to provide a sense amplifier having a controlled voltage swing.
It is a further object of the invention to provide a sense amplifier that has a fast operating speed.
It is another object of the invention to provide a sense amplifier that utilizes a minimal number of transistors in the voltage swing control circuit and wherein the transistors used are of a small size.
The above objects have been achieved by a sense amplifier having a pair of feedback paths between the sense amplifier output and the sense amplifier input for controlling the level of voltage swing on the sense amplifier output. The sense amplifier can be configured to operate in two different operating modes. In a first operating mode, the xe2x80x9cturboxe2x80x9d mode, both feedback paths are in operation. The first feedback path includes a transistor without threshold voltage enhancement and having a small W/L ratio in order to create the fastest possible sense operation while the second feedback path contributes stability to control the voltage swing on the sense amplifier output. In the second operating mode, the xe2x80x9cnon-turboxe2x80x9d mode, only the first feedback path is activated, which provides the maximum swing with a minor decrease in sensing speed. The first operating mode provides a higher margin a swing control, thus higher sensing speed, while the second operating mode allows for greater stability and consumes less power without compromising reliability. The invention allows the user to configure the sense amplifier to provide flexibility in meeting any requirements concerning the speed, operating margin, or power consumption of the sense amplifier.